Rotary rock drill bits have long been used in the petroleum and mining industries to drill for oil and minerals located in or below rock formations. These bits have also been employed by a different segment of the well drilling industry to drill wells for water. Many of the earth formations encountered during these operations are quite hard and subject the drill bit to severe stress. Moreover, the drill hole is usually quite narrow in diameter at the bottom in comparison to the diameter of the top, and this configuration subjects the bit to forces from several directions during the course of drilling operations. Proper lubrication of the bit's rotary elements and their bearing structures is essential if the bit is to perform optimally in such an environment.
The drill bits most suitable for this kind of drilling usually include three cutter cones rotatably mounted on journals attached to a bit body so that the axis of each cone is oriented near the center of the bit, and the teeth located in concentric rings on the face of each cone intermesh with the teeth on adjacent cones to provide the chipping and crushing action required to cut through the earth formation being drilled and create the drill hole. The typical bit body employed to support the cutter cones is rotatably attached to one end of a drill pipe and includes a body portion with three depending leg sections, each of which has bearing and support structure for rotatably mounting a cutter cone which is secured to a journal.
Viscous drilling "mud" is fed to the bit body through the drill string to cool the drill bit, which would otherwise be heated by the friction created during drilling. The mud is also used to wash the comminuted rock away from the drill bit cutting elements.
The drill bit cutting elements or cutter cones must rotate continuously during drilling operations to effectively drill through most rock formations. As a result, unless the bearings and support structures of the bit body are properly lubricated, the rotatably mounted cutter cones will not be able to rotate and will quickly become immobilized because of the frictional energy generated between the cone interiors and their supports. If this happens, the drill bit will not be capable of drilling. Consequently, the maintenance of optimally lubricated rotating cutter elements and their corresponding bearings is critical to rock drill bit performance. Even with proper lubrication, the average bearing life of available rock drill bits usually ranges from about 200,000 to about 570,000 revolutions, or 60 to 80 hours.
Moreover, even when the bearing surfaces of the rotating drill bit cutter elements are adequately lubricated, particulate materials, such as bits of rock and the like, may find their way to the cutter element bearing surfaces and into the lubricant. The presence of these contaminants in the lubricant could irreparably damage the bearing surfaces so that the cutting elements cannot rotate properly. For optimum drill bit operation, therefore, the drill bit cutting elements must not only be kept properly lubricated with an optimum supply of lubricant, but the lubricant must also be kept free of potentially damaging contaminants. Because the average rock drill bit bearing has a service life of only about 200,000 to 570,000 revolutions, the provision of adequate lubrication to the bearings and the elimination of wear-producing contaminants will increase bearing life and, therefore, the effective drilling time of the bit.
The prior art has proposed lubrication systems for rock drill bits which are designed either to provide an adequate supply of lubricant to the cutting element bearing surfaces, to reduce or eliminate particulate contaminants from the lubricant or both. However, the available systems, of which those disclosed in U.S. Pat. Nos. 4,167,220; 4,240,674; 4,412,590 and 4,446,933 are examples. These patents all disclose lubrication systems for rock or earth boring drill bits which suffer from limitations or disadvantages.
The lubrication system described in Pat. No. 4,446,933 to Bodine includes structure that rotates with the rotary cutting cones and uses the centrifugal force generated by rotation of the bit to drive particulate contaminants outwardly away from the bearings. A chamber divided by a floating piston in the bit body provides lubricant under pressure to the cone bearing surfaces in response to a pressure differential in the chamber created by the accumulation of drilling mud and water containing the contaminants on the opposite side of the piston from the lubricant. This system may effectively direct particulate contaminants away from the cone bearing surfaces as the bit rotates. However, the maintenance of an adequate supply of lubricant during bit operation is totally dependent on the accumulation of sufficient fluid in the chamber to produce the pressure differential to circulate lubricant to the cone. If the fluid passage between the cone and the fluid chamber becomes blocked with particulate contaminants, sufficient fluid will not be available to produce the necessary pressure differential.
Pat. No. 4,412,590 to Daly discloses an internal lubricant pump which includes an elastomeric member that moves across a grooved rigid surface during bit operation to circulate lubricant to the cone bearings. As the bit rotates, lubricant is pumped in a direction that depends on the configuration of the seal and grooves. This arrangement may provide an adequate supply of lubricant to the cone bearings when the seal and grooves have an optimum configuration. However, the system described in this patent will not effectively remove particulate contaminants from the lubricant. As a result, the groove configuration is likely to be eroded by the presence of hard particulate contaminants so that lubricant distribution will be adversely affected.
The "corkscrew" grooves for flushing lubricant past the bearings described in Pat. No. 4,167,220 does not reliably produce a pressure differential adequate to produce the kind of pumping action needed to keep all of the bearing surfaces fully lubricated. Lubricant is merely agitated and the flow is not forced. As a result, this system does not effectively remove particulate contaminants from contact with the cone bearings.
The drill bit cone lubricant assembly disclosed in Pat. No. 4,240,674 includes a complex arrangement of springs and other elements which function to force lubricant to the cone bearing structures. Although this system may keep the bearings adequately lubricated when all of the elements are functioning properly, the number of elements and complexity of their arrangement increases the likelihood that the harsh drilling environment will quickly damage components of this system. There is no provision in this system, moreover, for removing particulate contaminants.
Many of the known lubricant pumping elements for rock drill bits, moreover, are too delicate to withstand the high drill bit rotational revolutions per minute (rpms) or high lubricant viscosity typically encountered in drilling operations. Consequently, there is a need for a strong, but simple, lubrication system with a minimum number of moving parts for a rock drill bit that effectively provides lubricant to the cutter cone and its associated bearing surfaces during bit rotation. There is also a need for a simple lubrication system for a rock drill bit that additionally effectively removes particulate contaminants from the lubricant so that these contaminants do not contact and damage the cutter cone or other bit bearing surfaces.